Abstract: A THz-wave generator according to the present invention includes a nonlinear optical crystal 1 capable of parametric generation, a first laser device 12 for injecting a single-frequency first laser beam 7 as a pump wave into the nonlinear optical crystal, and a second laser device 14 for injecting a different single-frequency second laser beam 8 to thereby injection-seed a second laser beam 8 in a direction in which an idler wave is generated by the pump wave. By this configuration, it is possible, in generation of a THz-wave by use of the parametric effect in the nonlinear optical crystal under a non-collinear phase matching condition, to greatly increase a power of the THz-wave, to narrow its spectrum width, to make variable the wavelength of a generated THz-wave, and to hold its generation direction almost constant.